docs: update references to ghcr.io/renovatebot/renovate to v38

| datasource | package                      | from     | to      |
| ---------- | ---------------------------- | -------- | ------- |
| docker     | ghcr.io/renovatebot/renovate | 37.438.2 | 38.21.5 |

README.md

@@ -216,7 +216,7 @@ The Renovate version to use.
 If omitted the action will use the `latest` Docker tag.
 Check [the available tags on Docker Hub](https://hub.docker.com/r/renovate/renovate/tags).
 
-This sample will use `ghcr.io/renovatebot/renovate:37.438.2` image.
+This sample will use `ghcr.io/renovatebot/renovate:38.21.5` image.
 
 ```yml
 ....
@@ -229,7 +229,7 @@ jobs:
       - name: Self-hosted Renovate
         uses: renovatebot/github-action@v40.2.4
         with:
-          renovate-version: 37.438.2
+          renovate-version: 38.21.5
           token: ${{ secrets.RENOVATE_TOKEN }}
 ```
 
@@ -508,7 +508,7 @@ jobs:
         with:
           configurationFile: renovate.json5
           token: ${{ secrets.RENOVATE_TOKEN }}
-          renovate-version: 37.438.2
+          renovate-version: 38.21.5
         env:
           # This enables the cache -- if this is set, it's not necessary to add it to renovate.json.
           RENOVATE_REPOSITORY_CACHE: ${{ github.event.inputs.repoCache || 'enabled' }}